1. Field of the Invention
The present invention relates to an imaging apparatus and a method and program for controlling an imaging apparatus, and more particularly, to an imaging apparatus and a method and program for controlling an imaging apparatus, capable of providing an image in a highly stable manner in any environment of illumination, by correctly operating an infrared cutoff filter.
2. Description of the Related Art
In a monitor camera, a sensor of the camera generally has high sensitivity in an infrared range. Therefore, in a usual environment in which subjects have high illuminance, an infrared cutoff filter is inserted into an optical path to cut off an infrared component included in incident light thereby allowing it to obtain an image with proper color information.
On the other hand, in a dark environment in which subjects have low illuminance, the infrared cutoff filter is removed to achieve high sensitivity which allows it to obtain an image in such a dark environment, although color information is sacrificed. Japanese Unexamined Patent Application Publication No. 2000-224469 discloses a technique to automatically control the operation of inserting or extracting the infrared cutoff filter.
In the monitor camera disclosed in Japanese Unexamined Patent Application Publication No. 2000-224469, illuminance of a subject is calculated from an image signal output from an image sensing device, and the illuminance of the subject is compared with two threshold values (first reference illuminance and second reference illuminance (first reference illuminance<second reference illuminance)) to determine whether to insert or extract the infrared cutoff filter. More specifically, when the illuminance of the subject is lower than the first reference illuminance, the infrared cutoff filter is removed, while the infrared cutoff filter is inserted when the illuminance of the subject is higher than the second reference illuminance.
Referring to FIGS. 1 and 2, a further detailed explanation is given below as to the manner of determining whether to insert or extract the infrared cutoff filter of the monitor camera according to the conventional technique.
In an example shown in FIG. 1, a threshold value α1 and a threshold value α3 are set as the first reference illuminance and the second reference illuminance (such that threshold value α1<threshold value α3), and the insertion/extraction of the infrared cutoff filter is controlled in accordance with the illuminance value relative to the threshold values α1 and α3. In an example shown in FIG. 2, a threshold value α1 is set as the first reference illuminance, and a threshold value α2 is set as the second reference illuminance (threshold value α1<threshold value α2<threshold value α3). In the example shown in FIG. 1, there is a rather large difference between the first reference illuminance and the second reference illuminance. On the other hand, in the example shown in FIG. 2, there is a small difference between the first reference illuminance and the second reference illuminance.
In FIGS. 1 and 2, subject illuminance is represented along a vertical axis, and time passage is represented along a horizontal axis. “ON” denotes a state in which the infrared cutoff filter is inserted in an incident light path, and “OFF” denotes a state in which the infrared cutoff filter is pulled out of the incident light path. Solid lines represent a change in the illuminance of a subject with time, while broken lines represent apparent illuminance (that is, illuminance detected by the monitor camera in the state in which the infrared cutoff filter is pulled out).
First, the example shown in FIG. 1 is explained.
In the state in which the infrared cutoff filter is inserted in the incident light path, the monitor camera repeatedly checks whether the illuminance of the subject has decreased down to a value lower than the threshold value α1 set as the first reference illuminance. If the illuminance of the subject decreases beyond the threshold value α1 at a time T1, the monitor camera further determines whether the luminance of the subject remains lower than the threshold value α1 during a predetermined period of time (a period hatched in FIG. 1). If the illuminance of the subject has remained lower than the threshold value α1 during the predetermined period of time, then, at a time T2 after the predetermined period, the monitor camera pulls the infrared cutoff filter out of the incident light path.
The pulling of the infrared cutoff filter out of the incident light path causes the incident light to include an infrared component without being cut off. Thus, after a short waiting time has elapsed since the time T2 at which the infrared cutoff filter was extracted, an apparent illuminance value of the subject is detected by the monitor camera, and thus the detected illuminance becomes greater than the actual illuminance of the subject. Herein, the term “waiting time” denotes a time needed for an AE (Auto Exposure) mechanism to reach a stable state.
Thereafter, in the state in which the infrared cutoff filter is off the incident light path, the monitor camera continuously checks the illuminance of the subject (the apparent illuminance of the subject) to determine whether the illuminance of the subject has increased beyond the threshold value α3 set as the second reference illuminance.
That is, in the monitor camera in which the second illuminance (the threshold value α3) is set to a rather high value as in the example shown in FIG. 1, even when the illuminance of the subject becomes high enough as denoted by P in FIG. 1, the infrared cutoff filter is not inserted until the apparent illuminance of the subject becomes greater then the threshold value α3 set as the second reference illuminance.
If the illuminance of the subject becomes higher than the threshold value α3 at a time T3, the monitor camera continuously checks the illuminance of the subject to determine whether the illuminance of the subject remains higher than the threshold value α3 during a predetermined period of time. If it is determined that the illuminance of the subject has remained higher than the threshold value α3 during the predetermined period of time, then, at a time T4 after the predetermined period of time, the monitor camera inserts the infrared cutoff filter into the incident light path.
The insertion of the infrared cutoff filter at time T4 causes the infrared component to be cut off from the incident light. After a waiting time has further elapsed since T4, the illuminance of the subject detected by the monitor camera becomes equal to the actual illuminance of the subject.
In the state in which the infrared cutoff filter is again inserted in the incident light path, the monitor camera continuously checks the illuminance of the subject with reference to the threshold values α1 and α3 set as the first and second reference illuminance values in a similar manner as described above.
Now, the second example is described with reference to FIG. 2. When the monitor camera is in the state in which the infrared cutoff filter is inserted in the incident light path, the monitor camera continuously checks the illuminance of the subject to determine whether the illuminance of the subject has decreased beyond the threshold value α1 set as the first reference illuminance.
If the illuminance of the subject decreases beyond the threshold value α1 at a time T11, the monitor camera further checks the illuminance of the subject to determine whether the illuminance of the subject remains lower than the threshold value α1 during a predetermined period of time. If the illuminance of the subject has remained lower than the threshold value α1 during the predetermined period of time. The monitor camera pulls the infrared cutoff filter out of the incident light path at a time T12 after the predetermined period of time.
In this state in which the infrared cutoff filter is off the incident light path, the monitor camera continuously checks the illuminance of the subject to determine whether the illuminance of the subject increases beyond the threshold value α2 set as the second reference illuminance.
Note that as a result of the extraction of the infrared cutoff filter, after a time T13 at which a waiting time has further elapsed since the time T12, the illuminance of the subject detected by the monitor camera becomes the apparent illuminance of the subject which is higher than the actual illuminance of the subject. Therefore, in the example shown in FIG. 2, because the difference between the threshold value α1 and the threshold value α2 is small, the apparent illuminance of the subject can be higher than the threshold value α2 even if the actual illuminance of the subject remains lower than the threshold value α1.
Thus, in the example shown in FIG. 2, after the illuminance of the subject (the apparent illuminance of the subject) becomes higher than the threshold value α2 at a time T13, it is determined that the illuminance of the subject has remained higher than the threshold value α2 during a predetermined period of time. Thereafter, at a time T14 after the predetermined period of time, the monitor camera inserts the infrared cutoff filter into the incident light path.
In this state in which the infrared cutoff filter is inserted in the incident light path, the monitor camera continuously checks the illuminance of the subject to determine whether the illuminance of the subject decreases beyond the threshold value α1 set as the first reference illuminance. However, as a result of the insertion of the infrared cutoff filter, after a time T15 when a waiting time has elapsed since the time T14, the illuminance of the subject detected by the monitor camera becomes equal to the actual illuminance of the subject lower than the threshold value α1.
Thus, after the time T15, if the actual illuminance of the subject camera remains lower than the threshold value α1 during a predetermined period of time, it is determined that the illuminance of the subject detected by the monitor camera has remained lower than the threshold value α1 during the predetermined period of time, and thus at a time T16 after the predetermined period of time, the monitor camera pulls the infrared cutoff filter output of the incident light path.
Thus, the infrared cutoff filter is again brought into the extracted state, and the monitor camera continuously checks the illuminance of the subject to determine whether the illuminance of the subject increases beyond the threshold value α2 set as the second reference illuminance. However, as a result of the extraction of the infrared cutoff filter, after a time T17 when a waiting time has elapsed since the time T16, the apparent illuminance of the subject is again detected by the monitor camera, and thus the detected illuminance becomes higher than the actual illuminance of the subject.
Thus, at the time T17, the detected illuminance of the subject (the apparent illuminance of the subject) becomes higher than the threshold value α2, and thereafter it is determined that the illuminance of the subject has remained higher than the threshold value α2 during a predetermined period of time. Thus, at a time T18 after the predetermined period of time, the monitor camera inserts the infrared cutoff filter. Subsequently, the process described above is repeated.
In the second example, as described above, the effect of the increase in the apparent illuminance of the subject caused by the extraction of the infrared cutoff filter is not taken into account in the operation. That is, because there is only a small difference between the threshold value α1 set as the first reference illuminance and the threshold value α2 set as the second reference illuminance, the apparent illuminance of the subject can be higher than the threshold value α2 set as the second reference illuminance even when the actual illuminance of the subject is lower than the threshold value α1 set as the first reference illuminance. As a result, in the example shown in FIG. 2, the insertion and extraction of the infrared cutoff filter are repeated endlessly at intervals of the particular period needed to insert and extract the infrared cutoff filter.
In the conventional monitor camera, as described above, when the infrared cutoff filter is controlled in accordance with the two threshold values, the two threshold values or the difference therebetween can become improper depending on the environment or the subject. That is, it is difficult to properly set the threshold values and the difference therebetween, and this difficulty can cause an erroneous operation of the infrared cutoff filter.
For example, as described above with reference to FIG. 2, when the difference between the two threshold values is too small, if the illuminance of the subject is close to one of the two threshold values, there is a possibility that the insertion and extraction of the infrared cutoff filter is repeated endlessly, that is, hunting can occur. Conversely, when the difference between the threshold values is too large, there is a possibility that after the infrared cutoff filter is extracted, when the illuminance of the subject again becomes high enough as denoted by P in FIG. 1, the infrared cutoff filter is not re-inserted although the actual illuminance of the subject is high enough and the infrared cutoff filter should be re-inserted.